1. Field of the Invention
The present invention relates to a calcium sulfide oxidation method and apparatus for oxidizing calcium sulfide (CaS) generated at a power plant, etc., to thereby obtain calcium sulfate (CaSO4).
2. Description of the Prior Art
One example of a prior art oxidation apparatus for oxidizing CaS generated at a power plant, etc., into CaSO4 is shown in FIG. 10. In FIG. 10, numeral 1 designates an oxidation apparatus, numeral 1A designates a fluidized bed formed therein and numeral 1D designates a plenum. Numeral 6 designates a heat exchanger disposed in the oxidation apparatus 1, numeral 7 designates a cyclone and numeral 8 designates a particle distributor.
Numeral 9 designates a distributor plate disposed at a bottom portion of the oxidation apparatus 1. On this distributor plate 9, the fluidized bed 1A is formed, and limestone particles 100 containing char and CaS are supplied into the fluidized bed 1A through a nozzle 2. A mixture gas 101 of oxygen, steam and nitrogen is supplied into the plenum 1D through a nozzle 3. The mixture gas 101 is supplied into the fluidized bed 1A via the distributor plate 9 to vigorously effect a mixture combustion of the particles 100 in the fluidized bed 1A.
The oxygen concentration of a combustion gas 103 coming out of the fluidized bed 1A is set to 3 to 4% or more. Unless the oxygen concentration of 3 to 4% or more is maintained, it will be difficult to burn the char constantly. In the fluidized bed 1A, there occurs a reaction of CaS+202xe2x86x92CaSO4 between CaS and oxygen in the gas. While a large proportion of CaS is converted to CaSO4 as a whole in the fluidized bed 1A, CaS remains still within the particles.
The heat exchanger 6 is disposed in the fluidized bed 1A so that heat of the particles in the fluidized bed 1A is collected and a heating medium fluid 107 flowing in the heat exchanger 6 is heated. Combustion gas 108 coming out of the oxidation apparatus 1 enters the cyclone 7 to be separated into a dedusted combustion gas 109 and collected particles 110. The collected particles 110 are distributed by the particle distributor 8 into fine powder particles 111 to be extracted outside the system and coarse particles 112 to be returned into the fluidized bed 1A.
The coarse particles 112 are supplied into the fluidized bed 1A via a nozzle 5. Coarse particles 102, an ash content of the char, which are not pulverized in the fluidized bed 1A, but remain there so as not to be elutriated by the gas 103, are extracted outside the system via a nozzle 4 which is fitted to the distributor plate 9.
In the prior art apparatus as described above, there is contained in the particles 111 and 102 extracted outside the system a high concentration of CaS which has not been converted into CaSO4. This high concentration CaS contained in the particles 111 and 102 is gradually decomposed in the air to generate H2S, which results in the problem of an unfavorable influence being given to the environment.
Two reasons are considered why CaS remains in the particles discharged from the prior art oxidation apparatus. Firstly, CaSO4 generated on a particle surface at an initial stage of reaction forms a dense shell, so that oxygen is not supplied into the interior of the particle and CaS therein cannot react with oxygen. CaSO4, as compared with CaS, has a molecular volume of 1.8 times as larger, and as the reaction proceeds from CaS into CaSO4, gas diffusion pores existing in the particle clog, and oxygen cannot be supplied into the interior of the particle.
Secondly, a fine powder begins to entrain from the fluidized bed before ensuring sufficient reaction time required for complete oxidation of CaS, is discharged outside the oxidation apparatus as CaS contained in the fine powder, and is not yet completely oxidized.
Also, in case the fuel supply rate varies, because it is necessary to maintain the temperature and gas flow velocity in the fluidized bed within an appropriate range, it is preferable to change the heat transfer rate of heat transferred to the heating medium through the heat exchanger in the fluidized bed corresponding to the fuel supply rate.
In the prior art, however, it has been difficult to greatly change the heat transfer rate unless the height of the fluidized bed is changed. Further, in changing the fluidized bed height, it is necessary to put in or take out fluid medium to or from the fluidized bed, which work requires a great amount of time, and there has been a problem in that variations in the fuel supply rate cannot be followed well.
In view of the shortcomings in the prior art, as described with respect to the apparatus shown in FIG. 10, it is an object of the present invention to provide a CaS oxidation method and apparatus for oxidizing CaS into CaSO4 by which CaS particles can be oxidized into CaSO4 completely, as far as to the interior of the particle.
It is also an object of the present invention to provide an operation method of the CaS oxidation apparatus according to the present invention by which CaS can be efficiently oxidized into CaSO4.
In order to attain the object, the present invention provides the following oxidation method using an oxidation apparatus forming therein a first fluidized bed, a second fluidized bed on an outer side of the first fluidized bed and a space portion above the two fluidized beds.
That is, CaS-containing particles fluidized by a gas flow in the first fluidized bed are caused to collide violently with a heat exchanger or a baffle plate, disposed in the oxidation apparatus so as to traverse the gas flow. Accordingly, a dense shell of CaSO4 generated on a surface of the particle is abraded and oxygen is spread as far as to the interior of the particle to thereby accelerate an oxidation reaction of CaS into CaSO4. The baffle plate has no heat exchange function and has a surface coating applied thereto made of a material of a hardness higher than that of a fluid medium.
Further, a flow velocity in the space portion above the first fluidized bed is set to a terminal flow velocity or less of a fine powder entraining from the first fluidized bed to thereby cause the entraining fine powder soaring into the freeboard portion from the first fluidized bed to fall down into the second fluidized bed disposed on the outer side of the first fluidized bed.
Also, a gas flow velocity in the second fluidized bed is set lower than that in the first fluidized bed so that the fine powder that has fallen down into the second fluidized bed may not be entrained again. A volume of the second fluidized bed is set such that a particle residence time in the second fluidized bed becomes the value (or more) as calculated to a time required for complete oxidation of the fine powder, and the fine powder, containing CaS, which has been supplied from outside of the oxidation apparatus is supplied into the second fluidized bed.
Furthermore, an abrasion rate of the shell of CaSO4 in the first fluidized bed is controlled by a gas flow velocity in the first fluidized bed and an in-bed fill of the heat exchanger and baffle plate.
According to the CaS oxidation method described above, CaS is supplied into the second fluidized bed to be oxidized and is then sent to the first fluidized bed. In the first fluidized bed, CaS particles collide with the heat exchanger or baffle plate and the shell of CaSO4 generated on the surface of CaS particle is pulverized and abraded so that CaS particles are accelerated to be oxidized into CaSO4 as far as to the interior of the particle.
CaS particles so accelerated to be oxidized in the first fluidized bed soar into the space portion from the first fluidized bed and then fall down into the second fluidized bed, so that the CaS particles are oxidized into CaSO4 completely with a lower gas flow velocity and with sufficient time.
According to the CaS oxidation method of the present invention, CaS particles can be oxidized completely as far as to the interior of the particle and CaS can be prevented from being discharged outside the system.
Also, in order to attain the object, the present invention provides an oxidation apparatus of the following construction.
That is, the CaS oxidation apparatus according to the present invention is an oxidation apparatus that forms therein a first fluidized bed, a second fluidized bed on an outer side of the first fluidized bed and a space portion above the two fluidized beds, and comprises therein a partition for partially partitioning an interior of the oxidation apparatus into an inner side and an outer side.
The first fluidized bed is constructed on the inner side of the partition such that a heat exchanger or baffle plate is disposed in the first fluidized bed and particles received from the second fluidized bed on the outer side via a hole disposed below the partition are fluidized by an oxygen-containing oxidizing gas of air, oxygen or the like blown through a nozzle. At the same time, while a fuel of coal, coal char or the like supplied through a nozzle is burnt and CaS contained in the particles is oxidized, the particles are pulverized and abraded by the heat exchanger or baffle plate and a pulverized and abraded fine powder is sent to the freeboard portion, and a completely oxidized coarse powder is discharged outside the oxidation apparatus.
Also, the second fluidized bed is constructed on the outer side of the partition such that a desulfurizing agent of limestone, dolomite or the like supplied via a particle supply pipe and CaS contained in the particles that have fallen down from the space portion are fluidized to be oxidized into CaSO4 by an oxygen-containing oxidizing gas of air, oxygen or the like supplied via a nozzle.
Further, the space portion is constructed above the first fluidized bed and second fluidized bed such that a major part of the fine powder entrained from the first fluidized bed is caused to fall down into the second fluidized bed. Remaining particles are elutriated by a gas from the first fluidized bed and second fluidized bed to be discharged outside the oxidation apparatus.
The outer side of the partition where the second fluidized bed is formed is constructed by a portion where a heat exchanger for temperature control is disposed and a portion where no such heat exchanger is disposed.
According to the present invention, the CaS oxidation apparatus is partitioned in its interior into the inner side and the outer side, the first fluidized bed is formed on the inner side, the second fluidized bed is formed on the outer side and the particles which have been oxidized in the first fluidized bed on the inner side and entrained into the space portion thereabove fall down from the space portion into the second fluidized bed on the outer side to be completely oxidized there, and are then sent to the first fluidized bed to be extracted to the outside. In the first fluidized bed, the particles collide with the heat exchanger or baffle plate disposed in the first fluidized bed to be pulverized and abraded, and oxidation as far as to the interior of the particle is accelerated.
In the CaS oxidation apparatus according to the present invention, it is preferable for accelerating oxidation of particles containing less fine powder to make a height of the partition lower than that of a fluidized bed of the first and second fluidized beds, respectively.
That is, in case where a proportion of fine powder in the particles to be treated is small so that it is difficult to follow a partial load, a particle circulation rate of the particles moving into the second fluidized bed from the first fluidized bed over the partition can be increased.
Also, in the CaS oxidation apparatus according to the present invention, it is preferable for accelerating oxidation of the particles containing less fine powder to employ a construction such that a supply pipe of fuel of coal, coal char or the like is disposed on the outer side of the partition to thereby supply the fuel into the second fluidized bed on the outer side via a nozzle or, in addition thereto, a construction such that nozzles for supplying gas therethrough are distributed irregularly in the radial direction on the inner side of the partition.
That is, if the fuel is supplied into the second fluidized bed through a nozzle disposed on the outer side of the partition, then the second fluidized bed becomes a reduction atmosphere and CaO generated in the particle surface in that reduction atmosphere cause fine pores in the particle surface. Thus oxygen becomes liable to be spread through the fine pores as far as to the interior of the particle, and oxidation to the interior of the particle is accelerated.
Also, in addition to supplying the fuel on the outer side of the partition, nozzles for supplying the gas on the inner side of the partition are distributed irregularly in the radial direction to thereby form a portion of a reduction atmosphere in the radial direction below the first fluidized bed. CaO is then generated in the particle surface to thereby accelerate oxidation as far as to the interior of the particle, as mentioned above, in the first fluidized bed as well.
In the CaS oxidation apparatus according to the present invention, it is preferable for an efficient oxidation of CaS particles to do the following operation.
That is, a mean particle size of CaS-containing desulfurizing agent to be supplied is set in a range of 300 to 2000 xcexcm, firstly. Also, in order to control abrasion of the particles in the first fluidized bed, a fill of a heat exchanger or a baffle plate which has no heat exchange function is changed, and a gas flow velocity is changed to a range of 0.5 to 1.5 m/s. Further, a gas flow velocity in the second fluidized bed is set to a range of 0 to 1.2 m/s so as not to cause the pulverized particles to be entrained therein. A gas flow velocity in the space portion is changed to a range of 0.1 to 0.3 m/s to thereby control the amount entrained outside of the system. Also, the gas flow velocity in the first fluidized bed, the gas flow velocity in the second fluidized bed and the gas flow velocity in the space portion are changed to control a particle circulation rate from the first fluidized bed into the second fluidized bed. Further, in order to control a heat absorption rate of the entire CaS oxidation apparatus during variable fuel supply, an electrical signal is sent from a fuel supply control device to a flow control device of gas to be supplied into a portion where the heat exchanger of the second fluidized bed is disposed to thereby change the gas flow velocity in the portion to a range of 0 to 1.2 m/s.
By employing such operating condition as mentioned above, CaS can be oxidized into CaSO4 completely and efficiently by use of the apparatus according to the present invention.